Most communications devices such as routers, bridges, and switches need for their operation a stabilized DC voltage the temporal voltage fluctuation of which remains within predetermined limits. A stabilized DC voltage can be provided by DC-DC converters, AC-DC converters, or by batteries. In some cases a communications device may need an AC voltage meeting some predetermined requirements, which voltage can be provided by AC-AC converters and DC-AC converters. The power supply needs and requirements of a communications device are often specific to the installation site and even to the operational situation. The power demand of a communications device often depends on the number and transmission capacity of the communications links installed on the communications device, such as a router or switch, as well as on the communications protocols used. Additionally, the type of the electrical energy available can be different in different operating environments. In some operating environments there may be available a DC supply which gives a DC voltage from which a DC-DC converter produces the stabilized DC voltage needed by the communications device. In some other operating environments there may be available an AC supply which gives an AC voltage from which an AC-DC converter produces the stabilized DC voltage needed by the communications device.
In some prior-art solutions a communications device, such as a router or switch, is equipped with structures through which said communications device can be connected to one or more detachably installable power supply units. In such a case the power supply properties of the communications device can be flexibly altered by using power supply units suitable for different situations. Said power supply units are typically plug-in units which are plugged in the plug-in unit apertures of the communications device. If there exists a need to be able to connect the communications device to power supply units of many different types, the number of the different plug-in unit apertures as well as the space required for them often becomes a problem. For example, in communications devices installed on an equipment rack the surface area of the front panel is limited, which limits the number of plug-in unit apertures on said front panel.